I. Field of the Invention
The present invention relates to data communication. More particularly, the present invention relates to a novel and improved method and apparatus for high rate packet data transmission.
II. Description of the Related Art
A modern day communication system is required to support a variety of applications. One such communication system is a code division multiple access (CDMA) system which conforms to the xe2x80x9cTIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d, hereinafter referred to as the IS-95 standard. The CDMA system allows for voice and data communications between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both assigned to the assignee of the present invention and incorporated by reference herein.
In this specification, base station refers to the hardware with which the mobile stations communicate. Cell refers to the hardware or the geographic coverage area, depending on the context in which the term is used. A sector is a partition of a cell. Because a sector of a CDMA system has the attributes of a cell, the teachings described in terms of cells are readily extended to sectors.
In the CDMA system, communications between users are conducted through one or more base stations. A first user on one mobile station communicates to a second user on a second mobile station by transmitting data on the reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on the forward link of the same base station, or a second base station, to the second mobile station. The forward link refers to transmission from the base station to a mobile station and the reverse link refers to transmission from the mobile station to a base station. In IS-95 systems, the forward link and the reverse link are allocated separate frequencies.
The mobile station communicates with at least one base station during a communication. CDMA mobile stations are capable of communicating with multiple base stations simultaneously during soft handoff. Soft handoff is the process of establishing a link with a new base station before breaking the link with the previous base station. Soft handoff minimizes the probability of dropped calls. The method and system for providing a communication with a mobile station through more than one base station during the soft handoff process are disclosed in U.S. Pat. No. 5,267,261, entitled xe2x80x9cMOBILE ASSISTED SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein. Softer handoff is the process whereby the communication occurs over multiple sectors which are serviced by the same base station. The process of softer handoff is described in detail in U.S. patent application Ser. No. 08/763,498, entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATIONxe2x80x9d, filed Dec. 11, 1996, now U.S. Pat. No. 5,933,787, issued Aug. 3, 1999, by Klein S. Gilhousen et al., assigned to the assignee of the present invention and incorporated by reference herein.
Given the growing demand for wireless data applications, the need for very efficient wireless data communication systems has become increasingly significant. The IS-95 standard is capable of transmitting traffic data and voice data over the forward and reverse links. A method for transmitting traffic data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled xe2x80x9cMETHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSIONxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein. In accordance with the IS-95 standard, the traffic data or voice data is partitioned into code channel frames which are 20 msec wide with data rates as high as 14.4 Kbps.
A significant difference between voice services and data services is the fact that the former imposes stringent and fixed delay requirements. Typically, the overall one-way delay of speech frames must be less than 100 msec. In contrast, the data delay can become a variable parameter used to optimize the efficiency of the data communication system. Specifically, more efficient error correcting coding techniques which require significantly larger delays than those that can be tolerated by voice services can be utilized. An exemplary efficient coding scheme for data is disclosed in U.S. patent application Ser. No. 08/743,688, entitled xe2x80x9cSOFT DECISION OUTPUT DECODER FOR DECODING CONVOLUTIONALLY ENCODED CODEWORDSxe2x80x9d, filed Nov. 6, 1996, now U.S. Pat. No. 5,933,462, issued Aug. 3, 1999, by Andrew J. Viterbi et al., assigned to the assignee of the present invention and incorporated by reference herein.
Another significant difference between voice services and data services is that the former requires a fixed and common grade of service (GOS) for all users. Typically, for digital systems providing voice services, this translates into a fixed and equal transmission rate for all users and a maximum tolerable value for the error rates of the speech frames. In contrast, for data services, the GOS can be different from user to user and can be a parameter optimized to increase the overall efficiency of the data communication system. The GOS of a data communication system is typically defined as the total delay incurred in the transfer of a predetermined amount of data, hereinafter referred to as a data packet.
Yet another significant difference between voice services and data services is that the former requires a reliable communication link which, in the exemplary CDMA communication system, is provided by soft handoff. Soft handoff results in redundant transmissions from two or more base stations to improve reliability. However, this additional reliability is not required for data transmission because the data packets received in error can be retransmitted. For data services, the transmit power used to support soft handoff can be more efficiently used for transmitting additional data.
The parameters which measure the quality and effectiveness of a data communication system are the transmission delay required to transfer a data packet and the average throughput rate of the system. Transmission delay does not have the same impact in data communication as it does for voice communication, but it is an important metric for measuring the quality of the data communication system. The average throughput rate is a measure of the efficiency of the data transmission capability of the communication system.
It is well known that in cellular systems the signal-to-noise-and-interference ratio C/I of any given user is a function of the location of the user within the coverage area. In order to maintain a given level of service, TDMA and FDMA systems resort to frequency reuse techniques, i.e. not all frequency channels and/or time slots are used in each base station. In a CDMA system, the same frequency allocation is reused in every cell of the system, thereby improving the overall efficiency. The C/I that any given user""s mobile station achieves determines the information rate that can be supported for this particular link from the base station to the user""s mobile station. Given the specific modulation and error correction method used for the transmission, which the present invention seek to optimize for data transmissions, a given level of performance is achieved at a corresponding level of C/I. For idealized cellular system with hexagonal cell layouts and utilizing a common frequency in every cell, the distribution of C/I achieved within the idealized cells can be calculated.
The C/I achieved by any given user is a function of the path loss, which for terrestrial cellular systems increases as r3 to r5, where r is the distance to the radiating source. Furthermore, the path loss is subject to random variations due to man-made or natural obstructions within the path of the radio wave. These random variations are typically modeled as a lognormal shadowing random process with a standard deviation of 8 dB. The resulting C/I distribution achieved for an ideal hexagonal cellular layout with omni-directional base station antennas, r4 propagation law, and shadowing process with 8 dB standard deviation is shown in FIG. 10.
The obtained C/I distribution can only be achieved if, at any instant in time and at any location, the mobile station is served by the best base station which is defined as that achieving the largest C/I value, regardless of the physical distance to each base station. Because of the random nature of the path loss as described above, the signal with the largest C/I value can be one which is other than the minimum physical distance from the mobile station. In contrast, if a mobile station was to communicate only via the base station of minimum distance, the C/I can be substantially degraded. It is therefore beneficial for mobile stations to communicate to and from the best serving base station at all times, thereby achieving the optimum C/I value. It can also be observed that the range of values of the achieved C/I, in the above idealized model and as shown in FIG. 10, is such that the difference between the highest and lowest value can be as large as 10,000. In practical implementation the range is typically limited to approximately 1:100 or 20 dB. It is therefore possible for a CDMA base station to serve mobile stations with information bit rates that can vary by as much as a factor of 100, since the following relationship holds:                                           R            b                    =                      W            ⁢                                          (                                  C                  /                  I                                )                                            (                                                      E                    b                                    /                                      I                    o                                                  )                                                    ,                            (        1        )            
where Rb represents the information rate to a particular mobile station, W is the total bandwidth occupied by the spread spectrum signal, and Eb/Io is the energy per bit over interference density required to achieve a given level of performance. For instance, if the spread spectrum signal occupies a bandwidth W of 1.2288 MHz and reliable communication requires an average Eb/Io equal to 3 dB, then a mobile station which achieves a C/I value of 3 dB to the best base station can communicate at a data rate as high as 1.2288 Mbps. On the other hand, if a mobile station is subject to substantial interference from adjacent base stations and can only achieve a C/I of xe2x88x927 dB, reliable communication cannot be supported at a rate greater than 122.88 Kbps. A communication system designed to optimize the average throughput will therefore attempts to serve each remote user from the best serving base station and at the highest data rate Rb which the remote user can reliably support. The data communication system of the present invention exploits the characteristic cited above and optimizes the data throughput from the CDMA base stations to the mobile stations.
The present invention is a novel and improved method and apparatus for high rate packet data transmission in a CDMA system. The present invention improves the efficiency of a CDMA system by providing for means for transmitting data on the forward and reverse links. Each mobile station communicates with one or more base stations and monitors the control channels for the duration of the communication with the base stations. The control channels can be used by the base stations to transmit small amounts of data, paging messages addressed to a specific mobile station, and broadcast messages to all mobile stations. The paging message informs the mobile station that the base station has a large amount of data to transmit to the mobile station.
It is an object of the present invention to improve utilization of the forward and reverse link capacity in the data communication system. Upon receipt of the paging messages from one or more base stations, the mobile station measures the signal-to-noise-and-interference ratio (C/I) of the forward link signals (e.g. the forward link pilot signals) at every time slots and selects the best base station using a set of parameters which can comprise the present and previous C/I measurements. In the exemplary embodiment, at every time slot, the mobile station transmits to the selected base station on a dedicated data request (DRC) channel a request for transmission at the highest data rate which the measured C/I can reliably support. The selected base station transmits data, in data packets, at a data rate not exceeding the data rate received from the mobile station on the DRC channel. By transmitting from the best base station at every time slot, improved throughput and transmission delay are achieved.
It is another object of the present invention to improve performance by transmitting from the selected base station at the peak transmit power for the duration of one or more time slots to a mobile station at the data rate requested by the mobile station. In the exemplary CDMA communication system, the base stations operate at a predetermined back-off (e.g. 3 dB) from the available transmit power to account for variations in usage. Thus, the average transmit power is half of the peak power. However, in the present invention, since high speed data transmissions are scheduled and power is typically not shared (e.g., among transmissions), it is not necessary to back-off from the available peak transmit power.
It is yet another object of the present invention to enhance efficiency by allowing the base stations to transmit data packets to each mobile station for a variable number of time slots. The ability to transmit from different base stations from time slot to time slot allows the data communication system of the present invention to quickly adapt to changes in the operating environment. In addition, the ability to transmit a data packet over non-contiguous time slots is possible in the present invention because of the use of sequence number to identify the data units within a data packet.
It is yet another object of the present invention to increase flexibility by forwarding the data packets addressed to a specific mobile station from a central controller to all base stations which are members of the active set of the mobile station. In the present invention, data transmission can occur from any base station in the active set of the mobile station at each time slot. Since each base station comprises a queue which contains the data to be transmitted to the mobile station, efficient forward link transmission can occur with minimal processing delay.
It is yet another object of the present invention to provide a retransmission mechanism for data units received in error. In the exemplary embodiment, each data packet comprises a predetermined number of data units, with each data unit identified by a sequence number. Upon incorrect reception of one or more data units, the mobile station sends a negative acknowledgment (NACK) on the reverse link data channel indicating the sequence numbers of the missing data units for retransmission from the base station. The base station receives the NACK message and can retransmit the data units received in error.
It is yet another object of the present invention for the mobile station to select the best base station candidates for communication based on the procedure described in U.S. patent application Ser. No. 08/790,497, entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING SOFT HANDOFF IN A WIRELESS COMMUNICATIONS SYSTEMxe2x80x9d, filed Jan. 29, 1997, now U.S. Pat. No. 6,151,502, issued Nov. 21, 2000, by Roberto Padovani et al., assigned to the assignee of the present invention and incorporated by reference herein. In the exemplary embodiment, the base station can be added to the active set of the mobile station if the received pilot signal is above a predetermined add threshold and dropped from the active set if the pilot signal is below a predetermined drop threshold. In the alternative embodiment, the base station can be added to the active set if the additional energy of the base station (e.g. as measured by the pilot signal) and the energy of the base stations already in the active set exceeds a predetermined threshold. Using this alternative embodiment, a base station whose transmitted energy comprises an insubstantial amount of the total received energy at the mobile station is not added to the active set.
It is yet another object of the present invention for the mobile stations to transmit the data rate requests on the DRC channel in a manner such that only the selected base station among the base stations in communication with the mobile station is able to distinguish the DRC messages, therefore assuring that the forward link transmission at any given time slot is from the selected base station. In the exemplary embodiment, each base station in communication with the mobile station is assigned a unique Walsh code. The mobile station covers the DRC message with the Walsh code corresponding to the selected base station. Other codes can be used to cover the DRC messages, although orthogonal codes are typically utilized and Walsh codes are preferred.